The development of more efficient and lower noise power converters is a continuing goal in the field of power electronics . Power converters are typically employed in applications that require conversion (with isolation) of an input DC voltage to various other DC voltages, higher or lower than the input DC voltage. Examples include telecommunication and computer systems wherein high voltages are converted to lower voltages to operate the systems. Power converters often suffer from problems such as switching losses, switching noise and common-mode noise originating in the power transformer. Switching losses reduce system efficiency, resulting in greater input power requirements for the same output power. Switching and common-mode transformer noise, both conducted and radiated, require filtering to prevent or reduce interference with other sensitive electronic equipment.
Current power converter designs often implement one of two full bridge control strategies, namely, the conventional hard-switched, pulse-width modulated full bridge or the phase-shifted full bridge. Both control strategies employ a full bridge inverter topology having four controllable switches (e.g., power metal-oxide semiconductor field-effect transistors), an isolation transformer, an output rectifier and an output filter. The full-bridge inverter topology, with either control strategy, benefits from body diodes or separately added diodes antiparallel to the four controllable switches to accommodate currents in the parasitic inductances of the isolation transformer. A controller is included and employed to control the controllable switches.
The conventional full bridge generally operates as follows. The controllable switches are arranged in two diagonal pairs that are alternately turned on for a portion of a switching period to apply opposite polarities of the input DC voltage across a primary winding of the isolation transformer. The controllable switches thus operate to convert the input DC voltage into an AC voltage to operate the isolation transformer. Between conduction intervals of the diagonal pairs, all of the controllable switches are turned off for a fraction of the switching period. Ideally, this should force a voltage across the primary winding of the isolation transformer to zero. The output rectifier then rectifies the AC voltage from the isolation transformer. A rectified voltage of the isolation transformer should, therefore, ideally be a square wave with an average value proportional to a duty ratio of the diagonal pairs of controllable switches.
The output filter smooths and filters the rectified voltage to provide a substantially constant output voltage at the output of the power converter. The controller monitors the output voltage and adjusts the duty ratio of the diagonal pairs of controllable switches to maintain the output voltage at a constant level as the input DC voltage and the load current vary.
In practice, the rectified voltage is not a perfect square wave, however, because turning off all of the controllable switches induces a ring between a leakage inductance of the isolation transformer and a parasitic capacitance of the controllable switches. The ringing dissipates energy, thereby reducing the efficiency of the power converter. The ringing also gives rise to significant noise, such as conducted and radiated electromagnetic interference. In addition, current in the magnetizing inductance of the transformer should be reset, which is generally accommodated by the antiparallel diodes of the controllable switches.
The phase-shifted full bridge was developed to alleviate some switching loss and switching noise problems of the conventional full bridge. The construction of a power train of the phase-shifted full bridge is substantially similar to that of the conventional full bridge. Its advantages result, however, from the operation of the controllable switches to produce a substantially zero or reduced voltage across the controllable switches before the controllable switches are turned on. The phase-shifted full bridge operates by initially turning on both controllable switches of a diagonal pair (e.g., the top left and the lower right controllable switch). The phase-shifted full bridge may then turn off one controllable switch of the diagonal pair (e.g., the lower right switch) to begin the zero voltage period, instead of turning off both of the controllable switches. A controllable switch from the same leg (e.g., the upper right switch) is then turned on, allowing the current in the primary circuit to circulate through the two controllable switches with substantially zero or reduced voltage across the isolation transformer.
The two controllable switches thus clamp the voltage across the isolation transformer at about zero, thereby substantially eliminating the ringing behavior suffered by the conventional full bridge when the controllable switches are turned off. By clamping both ends of the primary winding of the isolation transformer to one rail and then, to the other rail, however, the phase-shifted full bridge induces a current flow through an intrinsic primary-to-secondary capacitance of the isolation transformer. As a capacitor potential is alternately charged from rail to rail, a common-mode noise is generated.
Furthermore, alternately circulating the primary current through the two top or bottom controllable switches may result in additional conduction losses. During the circulation intervals of the primary current, both the input current to the bridge and the output voltage applied to the output filter are substantially zero, requiring both input and output filtering.
An efficient application of the full bridge topology employs an unregulated full bridge operating at a substantially full duty cycle, with substantially simultaneous switching of the diagonal pairs of switches to provide reduced electromagnetic interference. The full bridge is followed by a post-regulator having a full range of regulation. To accommodate start-up and overload conditions, it may be necessary to operate the full bridge as a phase-shifted bridge.
Commonly used control integrated circuits, which are useful for providing phase-shifted operation, generally exhibit a number of critical limitations that complicate their use with full bridge topologies. The control integrated circuits are often unable to accurately provide full duty cycle operation. Further, the control integrated circuits may also be unable to provide substantially simultaneous switching of the diagonal switches of the full bridge when full duty cycle operation is required.
Accordingly, what is needed in the art is a controller for a power supply that overcomes the limitations of the prior art.